The Garmin Map Pilot navigation software and maps were always an option with the Audio20.
If your car never had this option, then a new Garmin Map Pilot SD-Card from the dealer will cost £499.
If your car had the Garmin Map Pilot option but the original SD-Card went missing, then you can purchase an Update SD-Card with the latest version of the software and maps from the dealer for £99.
There are also Garmin Map Pilot SD-Cards on eBay, they cost around £50 and are most likely fake, they might work in your car, but I have no idea. Some members here reported that the 'eBay Special' cards do work in the main, but the TomTom Live Traffic Data feature didn't work. But ultimately an eBay card will be a bit of hit and miss.